[ Feature Request ] NVENCC mux external audio file

Thanks for reading my topic.
NVENCC allow mux external file.
FastFlix allowing select external audio file is a good feature.
Have a nice week.
below the nvencc command for use external audio file

--audio-source [:{?}[;=...]/[]...]
Mux an external audio file specified.
file params
format=
Specify input format for the file.

input_opt=
Specify input options for the file.

track params
copy
Copy audio track.

codec=
Encode audio to specified audio codec.

profile=
Specify audio codec profile when encoding audio.

bitrate=
Specify audio bitrate in kbps.

samplerate=
Specify audio sampling rate.

delay=
Set audio delay in milli seconds.

dec_prm=
Specify params for audio decoder.

enc_prm=
Specify params for audio encoder.

filter=
Specify filters for audio.

disposition=
Specify disposition for audio.

metadata==
Specify metadata for audio track.

bsf=
Specify bitstream filter for audio track.

Examples

Example1: --audio-source ":copy"
Example2: --audio-source ":codec=aac"
Example3: --audio-source ":1?codec=aac;bitrate=256/2?codec=aac;bitrate=192;metadata=language=jpn;disposition=default,forced"
Example4: --audio-source "hw:1:format=alsa/codec=aac;bitrate=256"

Start by locating FastFlix's NVENCC option handling and the command construction for encoder arguments. Compare the requested --audio-source syntax and examples with the existing audio-selection flow; done means an external audio file can be selected and the resulting NVENCC command muxes it with the video.

Written by the indexing model from the issue text.